Leptospira interrogans serovar balcanica is a potential vector being investigated for spreading a biological control agent among introduced brushtail possums (Trichosurus vulpecula) in New Zealand. As previous studies have shown that possums are unlikely to contract leptospirosis through a contaminated environment alone, the objective was to determine whether L. interrogans serovar balcanica could be transmitted between sexually mature, socially housed possums. Possums were infected experimentally with L. interrogans serovar balcanica and housed in pairs or groups with uninfected possums for either 70 or 140 days, during the breeding or non-breeding seasons. No transmission occurred between any infected and uninfected possums during the non-breeding season. However, transmission occurred between females that had been socially housed in pairs or groups in the breeding season. Mixed sex transmission also occurred in pairs and groups, both from males to females and from females to males. Mixed sex transmission usually occurred rapidly (<44 days) and was not associated with the production of offspring. No transmission occurred between males during the breeding or the non-breeding seasons. Transmission probably occurs as a result of affiliative or sexual behaviour, but is unlikely to occur through fighting. The social transmission pathways determined in this study suggest that L. interrogans serovar balcanica may have the transmission attributes desired in a vector for biological control.